Radio direction finder



23, 1949 J. AlCARI-Jl 2,479,892

RADIO DIRECTION FINDER Filed July 3, 1945 s She ets-Sheet 1 Fig. 1 A

J. AICARDI RADIO DIRECTION FINDER Aug. 23, 1949.

5 Sheets-Sheet 5 Filed July 3, 1945 m IE in 3 gnmnxi kw ilk Jose biz Aicardi w, a? um 5. CM

Aug. 23, 1949. J. AICARDI 2,479,392

RADIO DIRECTION FINDER Filed July 3, 1945 5 Sheets-Sheet 4 Fig 13 N I HT F JOSep/Z Hie arm 1L aim I I J. AHCARDI 2,479,8

RADIO DIRECTION FINDER Filed July 5, 1945 5 Sheets-Sheet 5 Patented Aug. 23, 1949 UNETED ST RADIO DIRECTION FINDER Joseph Aicardi, Versailles, France, assignor to Sadir-Carpentier, Paris, France, a joint-stock company of France Application July 3, 1945, Serial No. 603,046 In France October 27, 1944 8 Claims.

A cos (coi l sin 0) and A cos wi+ sin 0) the phase difference between said electromotive forces being:

where 9 is th bearing of the transmitter, that is the angle between the direction of the transmitter and the perpendicular bisector of the line joining the two antennas.

One causes said tWo electromotive forces hav-- ing a pulsation w to beat with one and the same local electromotive force having a pulsation w,

The two beat electromotive forces obtained a frequency ww' also have the same phase difference 41rd g0- sin- 0 between them.

In these prior art systems, the two said electromotive forces having a frequency w-w' are then opposed in a circuit.

If the two beat electromotive forces are in phase and have the same amplitude, the resulting electromotive force is, of course, zero.

On the contrary, if both electromotive forces produced by the beat have a certain phase dif ference qr between them even if having the" same amplitude, th resulting electromotive force will have a certain value which is a function of the phase difierenoe.

Thus, the difierence' between the two beat elecs tromotive forces 0A and A? can be zero only when said electromotive force's are in phase and have the same amplitude.

Let us suppose that the voltage difference ob- 110h plitudes of the two electromotive force's present forces by an angle is zero. The. angle o by which said electromotive force has been displaced: in phase is necessarily equal to the initial phase difference between the two beat electromotive forces. Accordingly, the bearing 0 of the transmitter can be deduced from the knowledge of said dephasing angle and from the preceding formula go- Sill 9 However, said methods of the prior art present various inconveniences and more particularly the following ones:

The precise determination of the bearing angle 0- requires that one obtains a resultant which is rigorously zero for the suitable value of the dephasing imposed on one of the electromotive forces. Therefore, it is necessary that the ambe rigorously equal. Otherwise, the zero is replaced by a more or less broad minimum.

It is desirable to avoid this condition met in these systems of the prior art which it is always diificult to meet in a strict manner.

The present invention has more particularly for its object to remedy this inconvenience.

The present invention first relates to a direc-' tion finding method in which the electromotive forces induced by the transmitted signal observed:

having a pulsation w are caused to beat in two antennae A and B with one and the same localelectromotive force having a pulsation w and one combines in one and the same circuit the two so obtained beat electromotive forces having a pulsation w--w and between which there exists a dif ference of phase (p which is a function of the bearing of the transmitter, thi method being characterized in that one periodically reverses the phase of one of said beat electromotive forces so that the amplitude of the resulting combined electromotive force varies at the inversion frequency, the rate of the modulation in amplitude varying with the dephasing and becoming zero for a given dephasing and, accordingly, for a predetermined bearing, this determination which is' thus made by a null method being, owing to this fact, independent of the amplitude of the two combined electromotive forces.

The present invention also relates to another method corresponding to the one just mentioned and characterized in that one periodically varies the phase of one of the two beat electromotive forces of frequency ww so that in the course of tained by dephasin'g one of the electromotive 55 a period of variation of said phase, the dephaS- ing of the two beat electromotive forces of frequency w--w' passes thru or an odd multiple of that is the determination of the additional dephasing at this moment, defined by the reduction to zero of the modulation in amplitude of the resulting electromotive force determining the dephasing angle e and, accordingly, the direction of the transmitter the position of which is to be determined.

In a general manner the present invention relates to this method regardless of the particular apparatus used for carrying out the same. However, the present invention also covers apparatus making it possible to carry out the precedin method in a particularly efficacious manner and characterized in that the control station containing the observation apparatus such as a cathode-ray oscillograph, the local oscillator producing the auxiliary oscillation w adapted for beating with the received oscillation w, and the control panel is located at any point with respect to the antennae A, B, C and D and at any distance from the latter, said arrangement being possible owing to the fact that the measurement is unaffected by the variations of amplitudes, th only shift at the local high frequency w which is necessary comprising no condition of maintaining constant either the amplitude or the phase.

Furthermore, the present invention covers a device for the periodic variation of the phase characterized by network units formed of inductors connected in series and of capacities connected in shunt, or conversely, the inductors or the capacities of each of said units (or both together) bein periodically variable within narrow limits, so that the variation of the introduced dephasing is a linear one, the dephasing introduced by the artificial line comprising said units and representing the sum of the dephasings introduced by each of the network units being thus practically linear.

Besides, the present invention covers said device for the periodic variation of phase regardless of the systems in which said device is used.

Also the present invention relates to a method for the obtaining a voltage which is proportional to a dephasing, said method bein characterized in that one combines a voltage A the phase of which is periodically variable, with a voltage B0 in quadrature th mean value of the phase of the voltage 0A, the variation of amplitude of the resulting voltage BA being thus at every moment proportional to the dephasing when the variation of phase of the voltage 0A is small.

The methods and the apparatus used for carrying out the same which make it possible to attain the above mentioned objects have the characteristic features which appear from the following description and more particularly from the appended claims.

The direction finding method and apparatus according to the present invention are shown by way of example in the appended drawings, in which:

Figure 1 is a view showing the arrangement of the antennae.

Figure 2 is the vector diagram of two oscillation-s dephased by any angle Figure 3 represents the vector diagram of two oscillations dephased by 1r/2.

Figure 4 represents the oscillations resulting from the combining of two oscillations having the same frequency and one of which is periodically reversed.

Figure 5 represents the trace obtained in reception on a cathode-ray oscillograph showing phase relations wherein the horizontal sweep frequency is variably de-phased.

Figure 6 represents the arrangement of the four antennae for carrying out the method according to the present invention.

Figure '7 represents another trace obtained in reception on a cathode-ray oscillograph showing phase relations wherein the vertical sweep frequency is variably de-phased.

Figure 8 represents the superposition of the two traces of Figures 5 and 7.

Figure 9 represents a movable superposable screen facilitating the reading of the indications V of the oscillograph.

Figure 10 represents a special graduation of the screen of the oscillograph to provide direct reading.

Figure 11 is a plan view showing the arrangement of the five antennae of the antenna system according to the present invention.

Figure 12 is a circuit diagram of the direction finding system according to the present invention.

Figure 13 is a circuit diagram showing an artificial line with, a variable dephasing.

Figure 14 represents another form of an artificial line with a variable dephasing.

Figure 15 represents the vector diagram of the oscillations the resultant of which gives a voltage the variation of the amplitude of which is proportional to a value of dephasing.

Figure 16 is a circuit diagram of a third form of artificial line with a variable dephasing, and

Figure 17 is a block diagram showing the arrangement of the various units of the system of Figure 13.

According to the method which forms the subject matter of the present invention, one uses as heretofore for reception two antennae A, and '13 (Figure 1) and one causes the electromotive force induced in each of said antennae A and B and having the pulsation w to beat with a local oscilindicated.

One then combines the two beat electromotive forces say 65 and T3 which are thus obtained, by periodically reversing at the frequency f the phase of one of said beat oscillations, as, for in stance, K1 which at one instant t akes the direction which may be designated AB1, then at the next instant takes the opposite direction AB2, and

so on.

Then the resulting electromotive force passes at the inversion frequency from the value to the value O Bz= (Tl+ fi 2 and so on (Figure 2) where in general DB1 and 0B2 are not equal.

The amplitude of. the resulting electromotive force is the; same in. both alternations if the dephasing angle between A. and AB is equal to fi uregx Indeed, one sees in Figure 3. that 03;;032 if this particular dephasing relation exists.

Thus, the resulting electromotive force is generally modulated in amplitude at the. inversion frequency 1 (Figure. a). but the. modulation, rate is..reduced t o zerofgr a gephasing of thetwocom, P-onents 0A and AB1, ABz equal to whatever may be the ratio of the amplitudes;

The. dephasin-g. angle (9 between the two beat electromotiveforces' can be measured in the :fQl-.

lowing manner: One periodically varies at a irequency f the phase difference between the two beat elect-romotiveforces of frequency we n)". Let t be the variable dephasing which is. thus added for this-purpose at every instant to the dephasing angle (p of thetwo beat oscillations of frequency (n w which depha-sing angle (p is a. function of the direction of the transmitter.

The modulation rate of the resulting electro-- motive force is then zero for a value \PI of i// such that wherefrom q) the phase difference between the beat frequency oscillations and, accordingly c the hearing of the transmitter can be deduced using;

quation, 1 I

sin Ova-rying from 1 to +1 it must be possible to-vary the dephasing 191 from the value W 41rd the magnitude of the variation. of; 50-1 to be. obained.

increasing, of course, with d/A.

(Dnethen simultaneously traces, for instance on a cathode-ray oscillograph, the instantaneous values of the variable dephasing angle \l l. and the corresponding values of the resulting modulation rate-or, what is the same, the resulting oscillations which are detected and which have the frequencyf of the periodic inversions of the phase of' the beat frequency 0A.

The value 1,411 for which the modulationrate at inversion frequency J is zero indicates the value of" the complementary dephasing angle since at that moment 6. The value of 1hbeing thus known, sina is: deduced therefrom: by the formula:

When the delineation of the dephasin-g angle is eliectedon a cathode-ray oscillograph the pair of plates providing the horizontal sweeping is submitted to a voltage which is a function of the, variation at frequency 1" of t starting-from itsmean. value Sin 6=- while the vertical displacement of the spot, is produced by the oscillations having the frequency of inversion f.

Then, the screen of the oscillograph shows. the.

trace I of Figure 5.

This. trace is. formed of two envelopes l, and 2 which are symmetrical with respect to the axis mm of they screen.

These two envelopes intersect in a point. H for whichthe modulation rate of the oscillations having. the. pulsation to, that is to. say the. amplitude of the oscillations having the frequency f, is zero; The position of the point H with respect to. the point t} which. is the centre of the screen, indicates the value of the angle 1/11 and thus the value,

of the dephasin angle between the two beat frequency oscillations.

According to another form of the present inv vention, the preceding operations are effected By-applying the voltage which is a function of the variation of the dephasingangle 1/ to thepair of vertical plates of the. cathode-ray oscillograph, and the voltage: having the inversion fre--- quency f to the pair of horizontal plates, one obtains a trace 2 (Figure 7) in which the position.

of the point G defines 02.

Since the mean value of it is.

the Equations 2 and 3 may be written;

0- Km '0- Km S111. l m .7 ,00S4

G being the point of the axisof' the screen (with the centre ll) for which the envelope of the 0s cillations with the inversion frequency f intere sects th axi (Figure. 7).

Thus, the angle 0 which is the bearing of the transmitten, isigiven by its sine and its cosine.v

One thus obtains in like These two operationsof the antennae A,'B and C, D can be combined so as to give a direct reading of the angle 0.

For this purpose, one alternately effects changing over at a frequency f" the operations on the antennae A and B, on the one hand, and C and D, on the other hand, in order successively to obtain, on the screen of the cathode ray oscillograph at a first instant a trace similar to that of Figure 5, and at a subsequent instant a trace similar to that of Figure 7. The trace of Figure 5 gives sin 0, while the trace of Figure 7 gives cos 0. This alternation can be efiected'rather rapidly, for instance by means of an electronic change-over switch, in order that the two traces l and 2 may appear to coexist on the screen.

One then reverses with the same change over frequency ,f" the connection of the deflecting plates of the oscillograph so that in working with the antennae A, B for instance the deviation which is proportional to All/ is delineated in a, direction parallel to the axis W of the screen and the oscillations having the frequency f are delineated in a direction parallel to the axis XX while in the operation of the antennae C and D, the oscillations f are traced in a direction parallel to the axis YY', and the deviation proportional to Ad is traced in a direction parallel to the axis XX.

Under these conditions one obtains a superposition of the traces l and 2 (Figure 8) in which:

and where, accordingly, the direction HG represents the direction of the transmitter while the direction XX is the origin direction.

For reading the values of the line HG and the bearing angle 0 one sweeps over the screen of the oscillograph an orientable reference line 3 passing through the centre 0 of the screen (Figure 8), the periphery of this screen carrying graduations i which permit to read the bearing of the transmitter This reference line 3 can be traced advantageously on a transparent screen 80 and accompanied by portions of straight lines 31, 32, 33 and so on which are parallel and which will facilitate the directional adjustment of said movable screen about the centre 0 (Figure 9).

-With this method the amplitude readings at H and G will always be true zeros, which renders the reading precise.

On the other hand, with this method, it is sufficient that the electromotive force used for the sweeping (which electromotive force is at every moment proportional to the variation of the dephasing imposed on the one of the two beat electromotive forces coming from both antennae) be the same for both oscillograms in order that the direction HTS be correct.

The absolute value of the sweeping electromotive force does not matter. Likewise, the value of the amplification obtained for the oscillations having the inversion frequency f is without any importance, since only the position of the zero is of consequence.

On the contrary, if one wishes to read on each axis of the screen the value of the sine or of the cosine according to the considered axis of the bearing of the observed transmitter, it is necessary to take intoconsideration the factor For this purpose, for a given wave length A one arranges a small auxiliary transmitter the bearing of which is known and one adjusts the sweeping voltage so that the indications read on observing said transmitter are correct.

When the wave length of the transmission It is then possible to graduate the axes of the screen directly in values of the angle 6 which is the bearing of the transmitter. Of course, on the axis corresponding to sin 0, one reads at the same point the bearing angles 0 and 0 and on the axis corresponding to cos 0 the angles 0 and (3600). 7

Thus, one obtains a graduation array like that of Figure 10. One then immediately reads the bearing of the transmitter on the graduations, it is the number readat the same time at the two points where the amplitude of the oscillations is zero being the number 60 in Figure 10.

According to the value of the bearing angle 0, it is on one or the other of the two axes that the precision of the reading is the greatest, the number found on the axis which is less precise serving more particularly to remove ambiguity.

Figures 11 and 12 represent a system which is particularly suitable for carrying out the above described methods.

This system comprises four antennae A, B, C, D located on the corners of a square and an-auxiliary antenna E placed in the center of said square. This antenna E transmits the local oscillations with the frequency w which are received in agreement of phase by the four antennae A,

B, C, D, (Figure 11) At the foot of the antennae A and B are located the mixer devices 3| and 32 in each of which the beat frequence w-w' is produced (Figure 12).

The oscillations leaving the mixer 3| of antenna A with the pulsation ww' are transmitted thru a fixed dephasing device l5 and a dephaser l6 dephasing by the angle .1r/2 to the grid 20 of a triode 2| forming with a triode 22 a combining circuit 5.

Further referring to Fig. 12, the oscillations leaving the mixer 32 of antenna B with the pulsation ww' are transmitted thru a device l3 giving a dephasing variable periodically with the dephasing frequency f to a phase inversion switch l4 reversing the phase at the inversion frequency f. The mean value of the dephasing caused by the variable dephasing device [3 is equal to the value of the fixed dephasing caused by the device E5. The periodic variation of the phase of one voltage of frequency w--w' having a variable phase is effected, therefore, about a mean value of dephasing differing by 1r/2 from the fixed dephasing of the. other voltage of frequency ww'. The oscillations coming from the phase 9 inversion switch 14 are transmitted to the grid .23 of the triode 22 of the combiner 5.

Thus, one obtains on :the two parallel mounted plates .25 of the triodes 2| and 22 of the combiner circuit .5, the combination of the'two oscillations having the frequency w..w'.

The first oscillation of frequency (.w-w') derived from mixer 3! of the antenna A has a constant phase.

The second oscillation of frequency .(w-w') .de-

rived from mixer 32 of the antenna B has a phase 1// periodically variable at the dephasing frequency and reversed at the inversion frequency f, the means value of the phase difference between these two oscillations being equal to the phase difference on the output side of the mixers 31 and 32 increased by 1r/2.

Preferably, the periodic variation of the phase ,0 is at every moment proportional to the voltage V having the dephasing frequency f.

The variable dephasing circuit [3 is controlled I by an oscillator t delivering the dephasing frequency f.

The voltage of said oscillator 6 of frequency i also is applied to one of the two pairs of the plates of a cathode ray oscillograph 26. Thus,

this pair of plates is submitted to a voltage which is proportional to the variation of the dephasing angle 1/.

The other pair of plates of the cathode ray oscillograph receives the resulting oscillations having the frequency J of inversion coming from the combiner 5 and detected by the double diode 9. These indications derived from signals received by the antennae A and B are completed by the 1.

indications derived from signals received by the antennae C and D, a double change-over switch 1 alternately applying to a given pair of plates -of the oscillograph at the oscillograph changeover frequency f", first the voltage having the inversion frequency 1 coming from the antennae A and B and at the next instant the voltage having the inversion frequency 1 coming from the antennae C and D.

At these successive instants, one of these voltages is applied to the horizontal plates, and the other to the vertical plates of the oscillograph 24 (Fig. 12).

For this purpose, the antennae C and D are connected with circuits mounted in the same manner as the circuits of the antennae A and B respectively. Thus, at the foot of the antennae C and D are located the mixer devices SP and 3l 'in each of which the beat frequency w w' is produced.

The oscillations leaving the mixer 3| of the antenna having the frequency ww' are transmitted thru a dephasing device I5 for a "fixed dephasing, and a dephaser 16 producing a dephasing angle of 1r/2 to the combiner circuit 5 The oscillations leaving the mixer 32 of the antenna D with the pulsation w-w' are transmitted thru a device I3 giving a dephasing periodically variable at the dephasing fre quency 'f' to a phase change-over switchrld reversing the The variable dephasing circuit I3 is controlled by the oscillator 6 working with the dephasing frequency 1" which is common to both antenna groups A, B and C, D.

The successionof the images on-the-screenof *the cathode ray oscillograph 24 and their dephasing by 2 /2 on the screen are obtained by the combination of an oscillator of oscillograph change-over frequency f and 7 an electronic change-over switch 7 which is double.

For this purpose, an oscillator 8 applies a voltge of the inversion frequency f t th ole-bed cuits .of each of the p ase ch n ve witche 1A and 11,4.

Further referring to Fig. 12, the voltage V having the dephasing frequency f'from oscillator 6 is applied thru the medium of the potentiometer H to the grids 3e and of the triodes ill and 42 of the electronic change-over switch :1.

The voltage coming with the inversion frets quency J from the detector circuit 9 of the an- .triodeAS.

The :plate voltages of the four ,triodes All, 42,1011 .the one hand, and ill, .43 on the other hand, .of the double electronic change-over switch J, are alternately positive and negative at the .oscillograph change-over frequency f" and in opposition of phase.

second, thetriodesll and 4-2 the anodes of which carry a positive voltage transmit the oscillations of frequency coming from detector circuit "9 t0 the pair of plates 45, and the voltage having the de- {phasing frequency to the pair of plates 6 respectively; thus giving the trace with a horizontal axis characterizing 'the reception. of the antennae Aand B (Figure 5).

"During-the subsequent alternation of the same duration of the frequency f" the triodes 4!] and =42 cease-to operate, while the triodes 4i i and;4 3 the anodesof which carry a positive voltage transmit respectively the oscillations coming from the detector circuits of inversion frequency f to thepair of plates 4% and the voltage with the dephasing frequencyj' to the plate 45 respectively, thus giving the trace with a vertical axis characterizingthe reception -0n the antennae C and D Figured The observer who simultaneously observes the two so obtained traces determines therefrom the Each unit is, for instance, a -'n' network .unit :formed of one of the windings -52 of a trans- ;forn' er 5B withithree windings 5i, 52.and 5:3 and ,also. of condensers 54.

;Th e lastunitis terminated on a transformer'fifi for coupling with the. output. circuit to inverting switch-1'4.

The other ;two windings 5i and 53 of .each transformer ;are connected in series respec- The windings 5l, '5-2 and '53 of each transformer are wound on an iron core 56 or on a core made of compacted iron powder.

The windings 5! which are connected in series are connected with an electric source of direct current 51. a 1

The windings 53 connected in series are connected with the source or oscillator 6 of frequency f also delivering to an output resistance 58 the mid point of which is grounded.

The variable dephasing device l3 of Fig. 13 works as follows:

Each network unit 49 introduces into the artificial line a dephasing which is a function of the value of the self-inductance of the windings 52, the dephasage of the line being the sum of the dephasings introduced by each unit 49.

The winding 5! thru which the direct current flows fixes the mean value of the self-inductance 52 on a suitable portion of the magnetization curve, so that on both sides of said mean value the variation of the self-inductance is proportional to the variation of the current at least for small values of this variation.

The winding 53, thru which the alternating current having the dephasing frequency 7" flows, varies the self-inductance values of the windings on both sides of the mean value defined by the value of the direct current which flows thru the windings 5!. Thus, one obtains an artificial line in which the variation of the phase is the sum of the phase variations of each unit 49. Thus, said variation of phase is proportionalto the alternating current flowing thru the windings 53 with a frequency f. I V

The artificial line 15 or I5 introduced into the circuit receiving the oscillations of the antennae A or C, is identical with the previously described dephasing artificial line I3, but there is applied thereto to no alternating voltage. Thus, with the arrangement of Fig. 13, the dephasing introduced by the two artificial lines 13 and [5 between the currents which flow thru the same is due only to the voltage having a dephasing frequency 1" applied by the osillator source 6 to the alternating winding 53 of dephaser l3 so long as the variations of the direct current are not excessive.

According to another form of embodiment, the device for the periodic Variation of phase operating at the dephasing frequency can be obtained in the following manner (Figure 14).

The dephasing artificial line is again formed of a series of network units 49. These units are, for instance, 1r network units with inductance coils 52 connected in series and with capacities connected in shunt. Each of said capacities is formed of a fixed capacity and of a variable capacity H connected in parallel with the capacity 10. One periodically varies all the variable capacities ll in the same manner by gang control as by coupling their rotors, for instance, with a motor 12.

'In order to obtain, for the sweeping of the oscillograph, a voltage which is at every moment proportional to the dephasing which is produced, one can s till combine the periodically dephased tension CA, by any method, between the two extreme positions 0A and -OA with a. voltage BO in quadrature with the mean position of 0A. The variation of amplitude of the resulting voltage BA is at every moment proportional to the dephasing when the angle A 0A is small (Figure 15). If the variation of phase is obtained, for instance, by the rotation of variable small capacities H inparallel with the fixed capacities 'l'il of the retardation line (Figure 14) the voltage is combined at B, on the output side of the first network unit '49 with. a voltage in quadrature with the mean voltage at B (for instance the voltage at A if the units 49 are quarter wave units). A suiiicient number of units are 'connected in series for simultaneously meeting the conditions: a variation of phase which is very small for each network unit and a total variation of phase which is sufiiciently high.

It is also possible to adopt the phase variation' device of Figure 16.

This device is formed of network units 49 connected in series with an inductance coil 52 and a capacity 54. Eachof said units dephases on a mean value 1r/2. Variable reactan-ce valves I4 I4 M have their anodes connected with the input of each network unit 49 while their control grid 76 is connected thru a capacity H with the input of the preceding unit. Therefore, the current flowing thru each variable reactance valve is dephased by 7r/ 2 with respect to the voltage of the anode. Owing to this fact, each variable reactance valve M M 74 varies the equivalent capacity of each unit 49 by a quantity which is proportional to the voltage V applied to the grid 78 of each tube 14 so long as this variation is sufficiently small.

For obtaining aperiodic variation of phase each grid 18 is connected with an oscillator fi operating with the dephasing frequency Owing to this fact, the variation of phase is proportional to the voltage supplied by the oscillator 6.

The various parts of the above described direction finding system can advantageously be arranged in the following manner (Figure 17).

The control station 58, which can be considerably spaced from the antennae A, B, C, D, comprises the electronic change-over switch l with a cathode ray oscillograph 24, the oscillator .59 producing the oscillations w and the control panel Si], and serving also for starting the whole re" ceiving system (oscillators f and f, amplifiers and the like), as well as a remote control device for the tuning of the antennae and of their receiver; said tuning need not be very precise, It can be obtained, for instance, in a discontinuous 'manner provided that for each position of adjustment the circuits are tuned on the same frequency for all the receivers. Thus, with identical receiver circuits, no error of phase'is intro duced by adjustment differences.

Theonly precise adjustment to be effected is that of the local frequency w. This is very easy, the oscillator being placed in the control station.

At the foot of each of the four antennae A, B, C, D (or even in a place in the centre of the square), is located a detector device 3|, 32 (Fig. '12) which supplies and,-if necessary, amplifies the oscillations having the frequency w-w' produced by beating with the signals received on that antenna. According to the method of the present invention, these devices can have without any inconvenience very different amplifications.

The only condition to be met by them is that they must behave in the same manner as regards the dephasing of the oscillations w-w, i..e. of the beat oscillations, This condition is the more easily to be met the smaller ww' is. V

The other elements such as the phase change-- over switches l4 and M (Fig. 12), the combiner circuits 5 and 5 the detector circuits 9 and 9 the oscillators for the frequency f and for the frequency f", and the oscillator 6 for the freege'zasez l3 quency f, are located in an enclosure 6i located at the center of the square formed by the four antennae A, B, C, D (Fig. 17).

The control station 58 is connected with the enclosure i:

a) Thru conductors 52 carrying the oscillations having the inversion frequency f and a vari able amplitude coming from the detector circuits 9- and S and the voltage having the dephasi-ng frequency f" supplied by the oscillator 6;

(-b) By a line 63 delivering to the auxiliary antenna E in the center of the square formed by the antennae, the oscillations having the frequency m Thecontrol panel 58 is connected with the enclosure 6! by conductors 54.

Lines 65 and 65 transmit the currents having the frequency w-w' between the enclosure 3! and the antennae A, B and C, D respectively.

The lines 65' which carry the currents having the frequency wo' from the antennae A and B to the center are identical and have the same length. The same is true for the connections 6.6 between the center and the antennae C and D. Thus; no interference dephasing is introduced by these connections,

Thus, said arrangement makes it possible to control: the system from a station lying at any distance from the antenna.

Numerous modifications can be applied to the p above described plant without departing from the scope and spirit of the invention.

For instance, in case of the reception of vari able wave lengths, it may be desirable to cause the sweeping voltage automatically to vary when the received frequency varies.

For this: purpose, one modifies the frequency of the oscillation having the frequency w and beating with the received oscillation, so as to maintain constant the difference .uw', and one adjusts the potentiometers ll so as to vary the voltage according to the received frequency and according to a suitable law.

For this purpose, one can mechanically connect the control of the oscillation having the puisation w with the control of the potentiometer N. This is easy, since the oscillator 6' and thepotentiometer ii are both in the control station 58.

I-' claim:

1; In a radio direction finding system, a pair of spaced-rec tingantennas, a local oscillator; individual mixing means connected to each of said antennas picking up respectively the local oscillations from said local oscillator to produce a first and a second beat oscillation of same frequency but having a phase difference, phase adjusting means for adjusting the phase of one of said beat frequency oscillations, inverting means connected for periodically inverting by 180 at an inverting frequency the phase of one of said beat oscillations, combining means for combining said two beat frequency oscillations after said phase adjustments, detecting means for detecting the output of said combining means and detecting out said beat frequency, and measuring means for measuring the phase of the point of zero amplitude modulation at said inverting frequency of the output of said detecting means as a measure of the difference of phase of the initial dephasing of the signals received in said two antennas.

2. In a radio direction finding system, a pair of spaced receiving antennas, a local oscillator, individual mixing means connected to each of said antennas and picking up respectively the local--osc-illations from said local oscillator to produce a first and, a second beat oscillation both of the same beat frequency but having a phase difference, means for fixedly adjusting the phase of said first beat oscillation, means comprising an oscillator source of adjusting frequency for periodically varying at an adjusting frequency the phase of said second heat oscillation, inverting means connected for periodically inverting by at an inverting frequency the phase of said second beat oscillation after its said. variation: at saidv adjusting frequency, combining means for combining said: two beat frequency oscillations after their phases have been so modified, detecting means for detecting the output of said combining means and detecting out said beat frequency, and a cathode ray oscillograph having two pairs of plates, one of said: pairs of plates being; connected to the output of said detecting means, and the other of said pairs of plates being. connected to said adjusting frequency oscillator source, whereby the zero axis intercept of the: trace onsaid cathode ray oscillograph is a measure of the initial dephasing of the signals received in said two antennas and the: bearing of the transmitting source.

3. A system according toclaim 2', said means for periodically varyingat an adjusting frequency the phase: of. said second: heat oscillation further comprising'a plurality of capacity and inductance filter network units whose inductances are saturable ferromagnetic reactors adapted for periodic saturationby: the output of said oscillator source of adjusting frequency.

4. In. a radio direction finding: system, two pairs of spaced receiving. antennas respectively positioned on opposite corners of a square, alocal oscillator, mixing means connected to eachof said antennas and picking up respectively the local oscillations from sai'dlocal oscillator to pro ducefor. eachsaid: pair'of antennas a firstand'a second heat oscillation of the same beat frequency but of different phases, individual means for fixedlyadjust ing the phase of the first beat oscillation derived from each of said pairs of antennas; individual auxiliary means for pcriodically'varying at an adjusting frequency the phase of thesecondbeat oscillation v derived from each: of said pairs of antennas, said last mentioned auxiliary: means comprising a common oscillator. source" of' adjusting frequency connected to both of said auxiliary means for varying the phase of both of said second beat oscillations, individual inverting means for each of said pairs of antennas connected for respectively periodically inverting by 180 both at the same inverting frequency the phase of the second beat oscillation derived from each of said pairs of antennas after its said variation at said adjusting frequency, individual combining means for each said pair of antennas for combining respectively said two beat frequency oscillations derived from both of said pairs of antennas after their phases have been so modified, individual first and second detecting means for detecting the output of each of said combining means and detecting out the said beat frequency therefrom, a cathode ray oscillograph having two pairs of plates, and a cyclic oscillograph change-over switch operating at an oscillograph change-over frequency within the persistence of vision and adapted to and connected for connecting cyclically at a first instant, a first one of said pairs of plates to said first detecting means, and said second pair of plates to said adjusting frequency oscillator source, and at a subsequent instant connecting said first pair of plates to said adjusting frequency oscillator source and said second pair of plates to said second detecting means, whereby the zero axis intercepts of the traces on said cathode ray oscillograph are a measure of the initial dephasing of the signals received in said two pairs of antennas and the bearing of the transmitting source.

5. A system according to claim 4, said means for periodically varying at an adjusting frequency the phase of said second beat oscillation for each of said pairs of antennas further comprising for each of said pairs of antennas a plurality of capacity and inductance filter network units whose inductances are saturable ferromagnetic reactors adapted for periodic saturation by the output of said oscillator source of adjusting frequency.

6. A system according to claim 4, said means for periodically varying the phase of said second beat oscillation for each of said pairs of antennas further comprising for each of said pairs of antennas a plurality of capacity and inductance filter network units whose inductances are saturable ferromagnetic reactors adapted for periodic saturation by the Output of said oscillator source of adjusting frequency, and each of said saturable reactors further comprising a separate direct current saturating winding and an adjustable source of direct current connected to each of said direct current saturatin windings for adjusting the steady value saturation of said reactors.

7. In radio direction finding employing a pair of spaced receiving antennas, the method of getting a sharp visual direction indication which consists in beating the high-frequency signals received on each of said antennas with a local oscillation to derive therefrom two beat oscillations of the same frequency but different phase, varying the phase of a first one of said beat oscillations by a fixed phase difference, varying the phase of the second of said beat oscillations periodically at an adjusting frequency, periodically inverting by 180 at an inverting frequency the phase of the said second beat oscillation after said earlier modification of its phase, combining both of the so modified beat oscillations, detecting the so combined beat oscillations to detect out the beat frequency, tracing visually the outw put of said detecting means against said adjusting frequency as a base, and reading the zero axis at an adjusting frequency intercept of said trace as an indication of the direction of the transmitter of the signals received.

8. In radio direction finding employing two pairs of spaced receiving antennas respectively positioned on opposite corners of a square, the method of getting a sharp visual direction indication which consists in beating the high-frequency signals received on each of said antennas with a local oscillation to derive therefrom two pairs of beat oscillations of the same frequency but different phase, varying the phase of a first one of said beat oscillations of each of said pairs of oscillations by a fixed phase difference, varying the phase of the second one of said beat oscillations of both of said pairs of beat oscillations at the same adjusting frequency, periodically inverting by 180 at a common inverting frequency the phase of the second beat oscillation of each of said pairs of beat oscillations after said earlier modification of their respective phases, separately combining the two so modified beat oscillations of each of said pairs, separately detectin the so combined respective pairs of beat oscillations to detect out the beat frequency, and cyclically tracing successively at a change-over frequency which is within the persistence of vision two traces, the first said trace representing the resultant of detecting the combined said first pair of oscillations displayed as ordinates against said adjusting frequency displayed as abscissae, and the second said trace representing the said adjusting frequency displayed as ordinates against the resultant of detecting the combined said second pair of oscillations displayed as abscissae, and reading the zero axis intercepts of said two traces as an indication of the direction of the transmitter of the signals which are being received.

JOSEPH AICARDI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,968,068 Blancard et a1 July 31, 1934 2,007,099 Usselrnan July 2, 1935 2,160,135 Fritz May 30, 1939 2,232,592 Davies Feb. 18, 1941 2,408,121 Wirkler Sept. 24, 1946 2,408,122 Wirkler Sept. 24, 1946 

